Ulduar is a mysterious subterranean realm of ice and stone. This was once the storm giants' stronghold, but they didn't build it. Brann Bronzebeard believes it to be a titan city (it is also a production facility and prison), as it matched the design of the two temples in Northrend and other ruins dwarves found elsewhere (and because it contained the word "Uld"). Massive buildings protrude from the tops and sides of mountains, many hundreds of times taller than a human. When Brann Bronzebeard first explored the area, a section he discovered was covered in rock and ice. He thought it was an amazing place but couldn't tell from outside, because it was completely hidden to him. The exterior he saw looked like the other peaks to him: a jumble of rock and ice cut at angles only nature would have produced. The entrance he discovered was well-hidden and heavily guarded by crystal golems.[3] However large sections of the facility are now visible aboveground.

Inside, Ulduar is a profusion of tunnels and stairs and balconies. The entire place appears to be carved out of a massive cave, and many of the levels are natural ledges. It has the feeling, as if nature were improved upon rather than replaced or recreated. The surfaces are a strange mix of smooth and unaltered stone and ice, as when a master sculptor works in stone and lets the material’s natural texture add depth and beauty. It shows where the dwarves got their skill in stonework, but judging by Ulduar they're still a long way from matching their creators.

When Brann was exploring the place, the storm giants themselves were so few in number that one could wander the halls without ever meeting one — he believed this was probably for the best, since no one was supposed to gain entrance. The giants are a dying race desperately avoiding their fate. They seem to think another race will vanquish them some day, and so they hide in Ulduar rather than girding for battle. They are too frightened to battle the Scourge, but apparently they hate it. Ulduar is likely a treasure trove of information on the Titans and their activities.[3] Since then the storm giants have moved to the Howling Fjord.

In the RPG depending on the reference it is described as being a city, a titan research facility, a titan vault, or a combination of the above (in Lands of Mystery and other books). The text below reveals further information.

For millennia, Ulduar has remained undisturbed by mortals, far away from their concerns and their struggles. Yet since its recent discovery, many have wondered what the structure's original purpose may have been. Some thought it a city, built to herald the glory of its makers; some thought it a vault containing innumerable treasures, perhaps even relics of the mighty Titans themselves. Such speculations were wrong. Beyond Ulduar's gates lies no city, no treasure vault, no final answer to the Titan's mysteries. All that awaits those who dare set foot in Ulduar is a horror even the Titans could not, would not destroy, an evil they merely... contained.

Beneath ancient Ulduar, the Old God of death lies, whispering.... Tread carefully, or its prison will become your tomb.[4]

Ulduar is located at the very northern end of the Storm Peaks, making it the most northern location in all of Azeroth. After various quests it is revealed that Loken attacked the city with his iron dwarves and took it from the resident earthen.

Ulduar is possibly a reference to the god Uldar/Ulthar from H.P. Lovecraft's Cthuhlu mythos. Uldar was an "Elder God" (much like World of Warcraft's Titans) sent to Earth to keep vigil over the "Great Old Ones" (which World of Warcraft's Old Gods are based upon), much like how Ulduar is a prison for an Old God.